Solvent surface treatment of microporous polyurethane elastomer

ABSTRACT

A microporous polyurethane coating is given an appearance resembling that of calf grain leather by being sprayed with a solvent for the polyurethane, for example N-N dimethylformamide and heating the surface to evaporate the solvent. The preferred spray has a narrow range of droplet sizes defined in the specification.

United States Patent u [45] Sept. 5, 1972 SOLVENT SURFACE TREATMENT OFReferences Cited MICROPOROUS POLYURETHANE ELASTOMER 2 58 5 UN/TTEDSTATEbS PATENTS 6 l X ,8 71 11 1958 Dun ar ..2 448 [72] f f RWY NorthLyn", 2,926,389 3/1960 Garlington ..264/48 KmgsLynmNurfolk, England3,000,757 9/1961 Johnston ..264 /41UX 3,209,686 10/1965 Arthurs..264/341 X [73] Asslgnee' 1$?" 9 3,296,016 1/1967 2 Murphy ..264/4l uxg 3,301,935 1/1967 Stoeckhert ..264/48 X 22 Filed: Aug. 15,19 93,388,100 6/1968 Thoma ..264/4l UX [21] Appl' N07: 850666 PrimaryExaminer-Julius Frome Assistant ExaminerPaul A. Leipold Renaud U's'Apphcamm Dam Att0rneyWats0n, Cole, Grindle & Watson [63]Continuation-impart of Ser. No. 621,124,

March 7, 1967, abandoned. [57] ABSTRACT [30] F i Appligafinn p i it D Amicroporous polyurethane coating is given an ap- Mar. 7, 1966'"61656131116111; ;.';".".'..';;9'97*6/66 Pearanee resembling that of calfgrain leather by being sprayed with a solvent for the polyurethane, forexam- [52] us. C1 ..264/341, 117/161 KP, 156/2, p d e ylforma e andheating the surface 2 0/25 A 2 4 4 2 4/32 to evaporate the solvent. Thepreferred spray has a 51 1111.01 ..B29c 25/00 narrow range of dropletSizes defined the Specifica- [58] Field 61 Search ..264/48, 321, 341,41; 117/161;

7 Claims, No Drawings SOLVENT SURFACE TREATMENT OF MICROPOROUSPOLYURETHANE ELASTOMER This application is a continuation-in-part of myapplication Ser. No. 621,124 filed Mar. 7, 1967 and now abandoned.

The present invention relates to surface treatments for porous plasticsmaterials. It is concerned particularly, but not exclusively, withtreatments which give an artificial material a surface appearanceresembling natural leather.

According to the present invention a method of treating at least onesurface of a body, for example in the form of a sheet, of which at leastthe surface being treated consists of porous polymeric plasticsmaterial, involves the exposure of the surface to a solvent, thetreatment solvent, for the plastics material so as to partially collapsethe porous structure of the surface. The exposure may involve sprayingthe surface of the body.

In one form of the invention the exposure involves spraying the surfacewhile a current of air is blown across the surface between the surfaceand the source of the spray, the conditions being selected so that thegas permeability of the treated surface is till appreciably greater thanthat of the polymeric material when in a non-porous state. The surfacemay be moved relatively to the source of the spray, for example it maybe moved past the source of the spray. The source of the spray may be aspraygun utilizing air under pressure to form and propel the spray.

In one form of the invention the body may be made by a method, whichwill be referred to as a dunking method, which includes forming amixture comprising the plastics material dissolved in a solvent, shapingthe mixture, treating the shaped mixture with an inert liquid so as tocause the plastics material to come out of solution, and removing thesolvent and the inert liquid so as to form a porous body. The mixturemay include a removable solid filler and the inert liquid may be anonsolvent for the plastics material and may be used to remove thesolvent and the solid filler and may then be itself removed by heating.

In another form of the invention the body may be made by a method whichincludes forming a mixture comprising the plastics material dissolved ina solvent and a removable solid filler which is insoluble in thesolvent, shaping the mixture, heating the shaped mixture to remove thesolvent, treating the dried shaped mixture with a solvent for the fillerwhich is not a solvent for the plastics materials so as to remove thefiller, and drying the resultant shaped porous body, and the treatmentsolvent may contain a minor proportion of a polymeric plastics material,the lacquer working material.

In another form of the invention, the body may again be porous and maybe made by a method in which the plastic material containing removablesolid filler is applied by a melt roll coating process.

The surface may comprise a polyurethane and the treatment solvent may beN,N-dimethylformamide (referred to hereinafter as DMF).

The current of air may be heated, for example to between 40 and 100 C.

The term polyurethane is to be understood in its broadest sense andincludes any material derived from the reaction, between an isocyanate,such as a di-isocyanate, and a molecule, which will be called thepolyurethane precursor, which should generally be a polymeric molecule,containing at least two groups, such as hydroxyl, amide, or aminogroups, which contain hydrogen atoms capable of reaction with anisocyanate group. The poly-urethane precursor may be a polyesterderivative, or a polyether diol, or a polyester amide. I

The polyurethane used as the working material may be a thermoplasticelastomer having a low degree of cross-linking and thus may beapredominantly linear polymer. The molecular weight may be in the range20,000 to 300,000.

The polyurethane may be derived from a polyester such as the proprietarymaterial supplied by the B.F. Goodrich Chemical Company under the nameEstane or by Elastomer Products Limited (Elastollan Limited) under thename Elastollan. An example of another suitable polyurethane is theproprietary material supplied by Monsanto Chemicals Limited under thename Texin.

Polyurethanes of the type disclosed and claimed in German Pat.Specification No. 1189268 are particularly suitable.

Each of the polyurethanes referred to are derived from an essentiallylinear polyester containing hydroxyl groups. The polyester is producedby reacting together adipic acid and ethlene glycol and preferably has amolecular weight of approximately 2,000. The polyurethane is derivedfrom the polyester by reacting a minor proportion by weight of 1:4butylene glycol with a major proportion by weight of the polyester at anelevated temperature, both the polyester and the glycol being dehydratedbefore being reacted together. The mixture of polyester and 1:4 butyleneglycol is reacted with sufficient isocyanate to produce an essentiallylinear polyurethane, the isocyanate being 4:4 diphenyl methanedi-isocyanate and the reaction being carried out at elevatedtemperatures.

According to the manufacturer's published specifications the ElastollanTN6lEI-I90AK resin used in the following examples has the followingcharacteristics when compression moulded to produce a non-porousstructure such as a film or sheet (with further polymerizationoccurring, as is well known, during such moulding similar to thepolymerization which occurs when the material is dissolved in a solventsuch as dimethylformarnide, probably due at least in part to furtherreaction occurring between previously unreacted hydroxyl and isocyanategroups which are present in the elastomeric material), has a Shore Dhardness of 501- 3, a density of 1.23 g/cm"; a tensile strength of over450 Kg/cm, a modulus at percent elongation of Kg/cm, a modulus at 300percent elongation of Kg/cm an elongation at break of 450 percent (allas measured by DIN 53504); a Stosselastizitat of over 30 percent; a tearstrength (Streifen, DIN 53507) of 50 Kg/cm; and a tear strength (Graves,DIN 53515) of 90 Kg/cm. The Estane polyurethanes, which are mentionedabove, are described in the article by Stetz and Smith in Rubber AgeMay, 1965, pages 74 79 which refers to US. Pat. No. 2,871,218.

When the porous plastics material is one in which substantially all ofthe pores opening through the surface have a maximum dimension in theplane of the surface of less than 100 microns, the treatment solvent ispreferably caused to impinge on the surface in a finely divided form sothat substantially all of the droplets have a diameter between 1 and 30microns when they impinge on the surface. When between 60 and 70 of thedroplets have diameters in the range 1 to microns useful deepenings incolor and embossing effects tend to occur, so that the material takes onan appearance resembling natural calf grain leather.

The invention may be put into practice in various ways but threespecific embodiments will be described by ways of example.

In thev examples all parts are parts by weight.

The letters W.V.P. stand for water vapor permeability and the resultsare expressed in grams/sq. meter/24 hours and are determined by themethod described in British Standard Specification 3177/1959 but carriedout at 38 C with a nominal humidity gradient of 100 percent relativehumidity.

The hydrostatic head values are determined by the method described inBritish Standard Specification No. 2825. Nominal pore sizes areexpressed in microns and are determined by the method described inBritish Standard Specification No. 1752/1963 using n-propyl alcohol.Abrasion resistance, and flex life are measured by conventional methods.

In each example a replacement leather material suitable for use as agrain leather and made as described below has one surface treated inaccordance with the present invention.

The replacement leather material has a fibrous base layer, a tie layerin accordance with the present Applicants assignees co-pending BritishPat. Specification No. 9977/66 (Case P.P.l4) and a surface layer.

That British application has now issued as British Pat. No. 1148711. Itdiscloses the formation of a tie layer by applying a layer of a solutionof the polymer in a solvent which does not attack the polymer of thefibrous base layer and treating the layer of this solution so as to makeit microporous. Details of the formation of a specific tie layer on aspecific fibrous base layer are given below.

THE FIBROUS BASE LAYER This is made from a felt made from staple nylonfibers by mechanical entangling involving a needle punching process butno shrinking stage. This is impregnated with a linear polyester basedpolyurethane (Elastollan TN61 EH90AK) as a 12 percent by weight solutionin N,N-dimethylformamide (DMF). The impregnated felt is immersed inwater at C, for approximately 1 minute. The impregnated felt with thepolyurethane precipitated throughout its structure is left immersed inthe water which is pumped past the impregnated felt so that the felt iscontinuously contacted with fresh water for 2 hours. The product emergessubstantially free of all DMF and is dried at 90 C to produce asatisfactory supple product.

The felt before impregnation has the following properties:

Thickness 4.3 mms.

Weight in grams/meter 653 Weight in grams/meterlmm. thickness 141Density 0.9 grms/cc The following properties are all measured on aHoundsfield Tensometer.

Tensile strength extension percent and initial modulus are measured in acontinuous test on a single sample.

The material to be tested has an L direction, the direction of travel ofthe material during its formation, and an X direction, the direction atright angles to the L direction. For each material separate tests aremade in the L and X directions.

These are done on two samples 6 inches long and one-fourth inch wide cutfrom the material to be tested with the lengths of the samples parallelto the L and X directions of the material respectively.

The samples are mounted in the tensometer with their ends gripped in thejaws of the machine. The samples are then loaded to produce a constantrate of extension of 4 inches per minute.

The term Initial modulus (felt) used herein, is defined as the load inlbs/inch width of the sample/mm. thickness required to produce a 10percent extension in length under these loading conditions.

The term Tensile strength used herein, is defined as the load inlbs/inch width/mm. thickness at which the sample ruptures under theseloading conditions.

The term Extension percent as used herein, is defined as the percentincrease in length of the sample at the time of rupture.

Tear strength is measured on a specially shaped sample. As for the otherthree properties the samples are 6 inches long and one-fourth inch wideand are cut with their lengths in the L and X directions respectively.In addition a small notch is cut at the mid point of one side and asmall corresponding bulge is formed opposite the notch and extendingoutwards from the other side. The samples are in fact punched out of thematerial to be tested.

The notch propagates a tear in the test and the term Tear strength usedherein, is defined as the load in lbs/mm. thickness required to rupturethe sample Initial modulus (felt) L 1.13 X 0.56

Tensile strength L 24.6 X 55.0

Extension percent L 125 X 100 Tear strength L 8.9 X 14.6

THE TIE LAYER The fibrous base zone is split to 1 mm thick sheets with aband knife. The smooth surface thus produced is doctor knife coated witha layer of the following mixture using a gap setting of 0.015 inches.

TIE LAYER WORKING MATERIAL 25 A thermoplastic polyester basedpolyurethane, the proprietary material sold by B.F.Goodrich ChemicalCompany under the name ESTANE 5701 F. 1.

Solvent dimethylcyclohexanone Removable filler sodium chloride 75.

ground to a particle size range of 5-25 microns Pigment 0.6

The layer is then dried at C.

THE SURFACE LAYER The dried surface of the tie layer is doctor knifecoated with a layer of the following mixture using a gap setting of0.040 inches.

Surface working material 25 A thermoplastic polyester basedpolyurethane, the proprietary material sold by Elastollan Limited underthe name Elastollan TN 61 EH 98 A.K.

Solvent DMF 75 Removable filler sodium chloride 75 ground to a particlesize range of 5-25 microns Pigment 1.25

The composite material is then immersed in water at C for 20 minutes andthen washed with water at 80 C for 2 hours. The sheet is then dried at100 C.

This sheet has the following properties:

WVP 3500 Nominal pore size 7.2 microns Hydrostatic head 40 mm. Hg.

Abrasion resistance poor Flex life good.

EXAMPLE 1 The replacement leather material in the form of a long roll issupported on a stainless steel conveyor with its surface layer uppermostand carried at 2 ft/minute past a suitable spray-gun. The spray-gun isvertically above the sheet and points vertically downwards. The nozzleis 6 inches from the sheet. The spray-gun is arranged on a transverseslide and oscillated from side to side of the sheet at 50 passes perminute.

The treatment solvent may be NN, dimethyl formamide referred tohereinafter as DMF), and is supplied to the spray gun at a rate suchthat approximately 1 ml. per square foot of surface is used. Air at 16p.s.i. is also supplied to the spray gun.

Air is blown by suitable means counter current to the direction oftravel of the sheet between the sheet and the spray-gun and generallyparallel to the surface of the sheet. The velocity of the air adjacentthe spray is 800 cu. ft/minute and the air is heated to 60 C.

The sheet after passing the spray-gun immediately enters an oven whereit is heated at 120 C, to completely dry it.

The sheet has the following properties:

WVP 2900 Nominal pore size 4.0 microns Hydrostatic head 120 mm.Hg.

Abrasion resistance good Flex life good EXAMPLE 2 5 This is the same asExample 1 except that the treat- EXAMPLE 3 The sheet is first treatedidentically to Example 1.

The treated sheet is then retreated under the same conditions as Example2 except that the treatment solvent, DMF, is replaced by cyclohexanonecontaining lacquer working materials. This treatment solvent has thesame composition as that used in-Example 2, the DMF being replaced bycyclohexanone.

The sheet has the following properties:

WVP 2700 Nominal pore size 2.l microns Hydrostatic head mm. Hg.

Abrasion resistance good Flex life good It will be observed from theresults given that materials in accordance with the present inventionwhen compared with the untreated material have the followingadvantageous properties:

a. an increased resistance to the ingress of liquid water through thetreated surface.

b. the treated surface has an increased abrasion resistance.

c. the treated surface has a reduced pore size, reducing the soilingtendency of the material.

In Addition d. the color of the surface darkens and becomes less matt inappearance and may even achieve a glossy appearance and e. aleather-like break is achieved. By this is meant the appearance of thesurface when the sheet is bent with the surface zone innermost a largenumber of fine lines being temporarily formed.

By varying the spraying conditions and the treatment solvent for anyparticular porous plastics material the surface appearance can beselected to range from a smooth glossy finish to a mottled appearance.

To obtain the effects described herein some experimentation willgenerally be needed, since the rate of application of solvent, sprayingpressures, type and positioning of equipment, etc. will affect thecharacter of the product. For instance, it is obviously possible, byapplying sufficient solvent, to dissolve the entire microporous layer sothat a non-porous film remains on evaporation of the solvent, or toentirely collapse the porous structure throughout the microporous layer,rather than partially collapse it a the surface. With the teachingsherein of the effects to be attained, those skilled in the art will haveno difficulty in making the simple trials, with appropriate equipment,needed to 0 determine the best spraying conditions for that particularequipment and for the particular microporous sheet material andparticular solvent which is used.

The application of a stream of hot air to the solventsprayed sheet hasbeen found to make it much easier to obtain a high quality, generallyfiner grained, surface. One possible explanation for this is that theheat causes surface fusion of the solvent-carrying surface which has, ofcourse, a lower fusion temperature than the solvent-free material) andthat the evaporation occasioned by the high velocity air stream keepsthe solvent from penetrating into the microporous material so that thesolvent affects only the surface; this occurs even though thetemperature is, as will be evident, well below the boiling point of thesolvent.

It has been found that when the porous plastics material has been madeby a melt roll coating processor by a dunking method, and in particularby the method described in the examples, by selecting the sprayingconditions a surface finish closely approximating the surface finish,the socalled hair cell pattern, of natural calf leather can be obtained.

It will be appreciated that the smaller are the spray droplets producedby the spray-gun when they impinge on the surface the finer will be theindentations produced by them in the surface and the smoother thefinish.

In this connection high speed photography has revealed that when Example1 is carried out so that, of the droplets impinging on the surface, 67percent have a diameter between 1 and 10 microns, 28 percent have adiameter between 10 and 20 microns, and 5 percent have a diameterbetween 20 and 25 microns a very satisfactory product is obtained. Thecolor changes from light grey to a deep black and the surface takes onan appearance closely resembling that of a good quality calf grainleather.

An air pressure of 18 p.s.i. in Example 1 is found to produce thesedroplet size values and distributions when a Binks Bullows spray gunautomatic type L 500 is used with an air nozzle Type AlS, a materialnozzle Type A28 having a 28 thou. bore and a needle Type 28 occupyingthe bore.

If too low an air pressure is used the color change does not occur andthe surface does not take on the desired grain appearance since thedroplets coalesce and large uneven indentations are produced.

Thus if an air pressure of 5 p.s.i. is used, of the droplets impingingon the surface, none have a diameter between 1 and microns, 65 percenthave a diameter between 10 and 20 microns, and 35 percent have adiameter between 20 and 30 microns, and the surface does not resemble agrain leather.

If too high an air pressure is used, while the color change may occur,the surface does not take on the desired grain appearance sinceinsufficient indentation or embossing of the surface occurs.

Thus if an air pressure of 50 p.s.i. is used, of the droplets impingingon the surface, 79 percent have a diameter between 1 and 10 microns,about 20 percent have a diameter between 10 and 20 microns and less than1 percent have a diameter between 20 and 30 microns, and the surfaceonce more does not resemble a grain layer, though the color change maybe useful in achieving other finishes.

What I claim asmy invention and desire to secure by Letters Patent is:

1. A method of imparting an overall grain leatherlike appearance to asurface of flexible water vapor permeable sheet, said sheet including amicroporous soluble polyurethane elastomer surface zone overall,substantially all of the pores opening through said surface having nodimension in the plane of the surface greater than microns, said methodcomprising depositing a solvent for said polyurethane comprisingdimethylformamide on said surface overall but in finely divided liquidform and heating said surface to remove the deposited solvent byevaporation, said liquid solvent being applied to said surface when saidzone is dry and solid, said liquid solvent being applied in such finelydivided form and in such amount that the porous structure at saidsurface is partially collapsed by the treatment and said su faceigcreases in gloss and in resistance to ingress o l1qu1 watertherethrough but remains permeable to water vapor, said solvent being asolvent for said soluble polyurethane elastomer under the conditions ofthe process.

2. Process as in claim 1 in which said flexible sheet used as thestarting material has a colored surface and the color of said surface isdarkened and made less matt by said solvent deposition and heating.

3. Process as in claim 2 in which a spray of said solvent is directed atthe surface of the sheet and traversed back and forth across the surfaceof the sheet while the sheet moves transversely to the direction ofspraying and a current of heated air is blown at the sprayed surface ofthe sheet.

4. Process as in claim 3 in which the current of air is at a temperatureof 40 to 100 C.

5. Process as in claim 2, in which said heating is by a current ofheated air at 40 to 100 C., the polyurethane is a linear thermoplasticelastomer having a molecular weight in the range of 20,000 to 300,000and being soluble in dimethylformamide, and said deposited solventcomprises dimethylformamide.

6. Process as in claim 5 in which said elastomer is obtained by reactinga linear polyester having hydroxyl groups and a diisocyanate.

7. Process as in claim 5 in which said microporous soluble polyurethaneelastomer surface zone is one obtained, from a mixture of sodiumchloride particles of 5 to 25 micron particle size with a solution ofsaid polyurethane elastomer in dimethylformamide, by shaping saidmixture, contacting said mixture with water to cause the polyurethane tocome out of solution, to remove the dimethylformamide and to extract thesodium chloride, and then drying the shaped mixture to remove the water,said finely divided liquid being applied in such amount and such finelydivided form as to indent said surface.

2. Process as in claim 1 in which said flexible sheet used as thestarting material has a colored surface and the color of said surface isdarkened and made less matt by said solvent deposition and heating. 3.Process as in claim 2 in which a spray of said solvent is directed atthe surface of the sheet and traversed back and forth across the surfaceof the sheet while the sheet moves transversely to the direction ofspraying and a current of heated air is blown at the sprayed surface ofthe sheet.
 4. Process as in claim 3 in which the current of air is at atemperature of 40* to 100* C.
 5. Process as in claim 2, in which saidheating is by a current of heated air at 40* to 100* C., thepolyurethane is a linear thermoplastic elastomer having a molecularweight in the range of 20,000 to 300,000 and being soluble indimethylformamide, and said deposited solvent comprisesdimethylformamide.
 6. Process as in claim 5 in which said elastomer isobtained by reacting a linear polyester having hydroxyl groups and adiisocyanate.
 7. Process as in claim 5 in which said microporous solublepolyurethane elastomer surface zone is one obtained, from a mixture ofsodium chloride particles of 5 to 25 micron particle size with asolution of said polyurethane elastomer in dimethylformamide, by shapingsaid mixture, contacting said mixture with water to cause thepolyurethane to come out of solution, to remove the dimethylformamideand to extract the sodium chloride, and then drying the shaped mixtureto remove the water, said finely divided liquid being applied in suchamount and such finely divided form as to indent said surface.